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What causes antiphospholipid syndrome (APS)?

Antiphospholipid syndrome (APS) is an autoimmune disease, which means that it’s caused by your immune system attacking healthy parts of the body instead of fighting infections.

If you have APS, your immune system produces harmful antibodies called antiphospholipid antibodies (aPL) which attack proteins are linked to fats in your body. The most important of these proteins is called beta-2-glycoprotein I. When aPL stick to this protein they can interfere with blood cells. The cells change in such a way that the blood becomes sticky and more likely to clot inside the vessels. In a pregnant woman aPL can also affect the cells of the womb and the placenta, which can make the baby grow more slowly and increase the risk of miscarriage.

Although people with APS have a higher risk of thrombosis than other people, they may go for many years without suffering clots. Occasionally, the thrombosis occurs during an infection such as a sore throat; however, in the vast majority of people the thrombosis comes ‘out of the blue’. The risk of blood clots can be reduced by blood-thinning drugs and by reducing other factors that can cause clots, including:

  • smoking
  • immobility (related, for instance, to the thrombosis seen after long-haul flights)
  • the contraceptive pill
  • genetic factors – there may be a family history of clots, miscarriages, other autoimmune diseases such as lupus or thyroid problems.


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